This type of hybrid system provided with an engine, two rotary machines, and a power distribution mechanism (planetary gear mechanism) is conventionally known. In the hybrid system, a rotary shaft of the engine, a rotary shaft of a first rotary machine, a rotary shaft of a second rotary machine, and a driven wheel are connected to respective rotational elements of the power distribution mechanism. Following Patent Literature 1 discloses a hybrid system in which a differential device formed of a pair of first and second planetary gear mechanisms, a clutch, and two brakes are interposed between the rotary shaft of the engine and the rotational element of the power distribution mechanism. The differential device is used as a transmission which changes a speed of rotation of the engine. One engaging unit of the clutch is connected to the rotary shaft of the engine and a carrier of the first planetary gear mechanism and the other engaging unit is connected to a ring gear of the first planetary gear mechanism. The carrier and a sun gear of the first planetary gear mechanism are connected to a sun gear and a ring gear of the second planetary gear mechanism, respectively. The sun gear of the first planetary gear mechanism and the ring gear of the second planetary gear mechanism are connected to a carrier of the power distribution mechanism. A first brake may stop rotation of the ring gear of the first planetary gear mechanism and the other engaging unit of the clutch. A second brake may stop rotation of a carrier of the second planetary gear mechanism. In the hybrid system, engaging the clutch and disengaging each brake make an underdrive mode (UD mode) at the time of middle load and high load, disengaging the clutch and the second brake and engaging the first brake make an overdrive mode (OD mode) at the time of light load, and disengaging the clutch and the first brake and engaging the second brake make a rearward movement mode.